There's a big void in the top of a mountain. Behind the abandoned truck task at the lake.
Flooded foothills mountain void
@bushwacker I think I found that same void yesterday...
@lun_wing Here is a screenshot of the hole I found, not positive, but it may be the same as what Bushwacker saw. It looks like a texture glitch in the picture, but when you pan the camera over it, it's a hole straight down. I found it trying to circle around to activate the watchtower in the northwest portion of the map (as you can see the tower in the very right hand side of the picture)